Metrology apparatus

ABSTRACT

A metrology tool for determining a parameter of interest of a structure fabricated on a substrate, the metrology tool comprising: an illumination optical system for illuminating the structure with illumination radiation under a non-zero angle of incidence; a detection optical system comprising a detection optical sensor and at least one lens for capturing a portion of illumination radiation scattered by the structure and transmitting the captured radiation towards the detection optical sensor, wherein the illumination optical system and the detection optical system do not share an optical element.

FIELD

The invention relates to a metrology apparatus and associated methodsfor determining a parameter of a structure fabricated on a substrate.More specifically, the invention may relate to a metrology apparatususing a computational imaging methodology. In some examples, theinvention may relate to determining a parameter relating to overlay.

BACKGROUND

A lithographic apparatus is a machine constructed to apply a desiredpattern onto a substrate. A lithographic apparatus can be used, forexample, in the manufacture of integrated circuits (ICs). A lithographicapparatus may, for example, project a pattern (also often referred to as“design layout” or “design”) at a patterning device (e.g., a mask) ontoa layer of radiation-sensitive material (resist) provided on a substrate(e.g., a wafer). The projected pattern may form part of a process tofabricate a structure onto the substrate.

To project a pattern on a substrate a lithographic apparatus may useelectromagnetic radiation. The wavelength of this radiation determinesthe minimum size of features which can be formed on the substrate.Typical wavelengths currently in use are 365 nm (i-line), 248 nm, 193 nmand 13.5 nm. A lithographic apparatus, which uses extreme ultraviolet(EUV) radiation, having a wavelength within the range 4-20 nm, forexample 6.7 nm or 13.5 nm, may be used to form smaller features on asubstrate than a lithographic apparatus which uses, for example,radiation with a wavelength of 193 nm.

Low-k₁ lithography may be used to process features with dimensionssmaller than the classical resolution limit of a lithographic apparatus.In such process, the resolution formula may be expressed as CD=k₁×λ/NA,where λ is the wavelength of radiation employed, NA is the numericalaperture of the projection optics in the lithographic apparatus, CD isthe “critical dimension” (generally the smallest feature size printed,but in this case half-pitch) and k₁ is an empirical resolution factor.In general, the smaller k₁ the more difficult it becomes to reproducethe pattern on the substrate that resembles the shape and dimensionsplanned by a circuit designer in order to achieve particular electricalfunctionality and performance. To overcome these difficulties,sophisticated fine-tuning steps may be applied to the lithographicprojection apparatus and/or design layout. These include, for example,but not limited to, optimization of NA, customized illumination schemes,use of phase shifting patterning devices, various optimization of thedesign layout such as optical proximity correction (OPC, sometimes alsoreferred to as “optical and process correction”) in the design layout,or other methods generally defined as “resolution enhancementtechniques” (RET). Alternatively, tight control loops for controlling astability of the lithographic apparatus may be used to improvereproduction of the pattern at low k1.

In lithographic processes, it is desirable to make frequentlymeasurements of the structures created, e.g., for process control andverification. Tools to make such measurements are typically calledmetrology tools MT. Different types of metrology tools MT for makingsuch measurements are known, including scanning electron microscopes orvarious forms of scatterometer metrology tools MT. Scatterometers areversatile instruments which allow measurements of the parameters of alithographic process by having a sensor in the pupil or a conjugateplane with the pupil of the objective of the scatterometer, measurementsusually referred as pupil based measurements, or by having the sensor inthe image plane or a plane conjugate with the image plane, in which casethe measurements are usually referred as image or field basedmeasurements. Such scatterometers and the associated measurementtechniques are further described in patent applications US20100328655,US2011102753A1, US20120044470A, US20110249244, US20110026032 orEP1,628,164A, incorporated herein by reference in their entirety.Aforementioned scatterometers may measure gratings using light from softx-ray and visible to near-IR wavelength range.

Scatterometers MT may be angular resolved scatterometers. In such ascatterometer reconstruction methods may be applied to the measuredsignal to reconstruct or calculate properties of the grating. Suchreconstruction may, for example, result from simulating interaction ofscattered radiation with a mathematical model of the target structureand comparing the simulation results with those of a measurement.Parameters of the mathematical model are adjusted until the simulatedinteraction produces a diffraction pattern similar to that observed fromthe real target.

Scatterometers MT may alternatively be spectroscopic scatterometers MT.In such spectroscopic scatterometers MT, the radiation emitted by aradiation source is directed onto the target and the reflected orscattered radiation from the target is directed to a spectrometerdetector, which measures a spectrum (i.e. a measurement of intensity asa function of wavelength) of the specular reflected radiation. From thisdata, the structure or profile of the target giving rise to the detectedspectrum may be reconstructed, e.g. by Rigorous Coupled Wave Analysisand non-linear regression or by comparison with a library of simulatedspectra.

Scatterometers MT may alternatively be ellipsometric scatterometer. Theellipsometric scatterometer allows for determining parameters of alithographic process by measuring scattered radiation for eachpolarization states. Such metrology apparatus emits polarized light(such as linear, circular, or elliptic) by using, for example,appropriate polarization filters in the illumination section of themetrology apparatus. A source suitable for the metrology apparatus mayprovide polarized radiation as well. Various embodiments of existingellipsometric scatterometers are described in U.S. patent applicationSer. Nos. 11/451,599, 11/708,678, 12/256,780, 12/486,449, 12/920,968,12/922,587, 13/000,229, 13/033,135, 13/533,110 and 13/891,410incorporated herein by reference in their entirety.

In one embodiment of the scatterometer MT, the scatterometer MT isadapted to measure the overlay of two misaligned gratings or periodicstructures by measuring asymmetry in the reflected spectrum and/or thedetection configuration, the asymmetry being related to the extent ofthe overlay. The two (typically overlapping) grating structures may beapplied in two different layers (not necessarily consecutive layers),and may be formed substantially at the same position on the wafer. Thescatterometer may have a symmetrical detection configuration asdescribed e.g. in co-owned patent application EP1,628,164A, such thatany asymmetry is clearly distinguishable. This provides astraightforward way to measure misalignment in gratings. Furtherexamples for measuring overlay error between the two layers containingperiodic structures as target is measured through asymmetry of theperiodic structures may be found in PCT patent application publicationno. WO 2011/012624 or US patent application US 20160161863, incorporatedherein by reference in its entirety.

Other parameters of interest may be focus and dose. Focus and dose maybe determined simultaneously by scatterometry (or alternatively byscanning electron microscopy) as described in US patent applicationUS2011-0249244, incorporated herein by reference in its entirety. Asingle structure may be used which has a unique combination of criticaldimension and sidewall angle measurements for each point in a focusenergy matrix (FEM—also referred to as Focus Exposure Matrix). If theseunique combinations of critical dimension and sidewall angle areavailable, the focus and dose values may be uniquely determined fromthese measurements.

A metrology target may be an ensemble of composite gratings, formed by alithographic process, mostly in resist, but also after etch process forexample. Typically the pitch and line-width of the structures in thegratings strongly depend on the measurement optics (in particular the NAof the optics) to be able to capture diffraction orders coming from themetrology targets. As indicated earlier, the diffracted signal may beused to determine shifts between two layers (also referred to ‘overlay’)or may be used to reconstruct at least part of the original grating asproduced by the lithographic process. This reconstruction may be used toprovide guidance of the quality of the lithographic process and may beused to control at least part of the lithographic process. Targets mayhave smaller sub-segmentation which are configured to mimic dimensionsof the functional part of the design layout in a target. Due to thissub-segmentation, the targets will behave more similar to the functionalpart of the design layout such that the overall process parametermeasurements resembles the functional part of the design layout better.The targets may be measured in an underfilled mode or in an overfilledmode. In the underfilled mode, the measurement beam generates a spotthat is smaller than the overall target. In the overfilled mode, themeasurement beam generates a spot that is larger than the overalltarget. In such overfilled mode, it may also be possible to measuredifferent targets simultaneously, thus determining different processingparameters at the same time.

Overall measurement quality of a lithographic parameter using a specifictarget is at least partially determined by the measurement recipe usedto measure this lithographic parameter. The term “substrate measurementrecipe” may include one or more parameters of the measurement itself,one or more parameters of the one or more patterns measured, or both.For example, if the measurement used in a substrate measurement recipeis a diffraction-based optical measurement, one or more of theparameters of the measurement may include the wavelength of theradiation, the polarization of the radiation, the incident angle ofradiation relative to the substrate, the orientation of radiationrelative to a pattern on the substrate, etc. One of the criteria toselect a measurement recipe may, for example, be a sensitivity of one ofthe measurement parameters to processing variations. More examples aredescribed in US patent application US2016-0161863 and not yet publishedU.S. patent application Ser. No. 15/181,126, incorporated herein byreference in its entirety.

There is a growing need of improving the capabilities of thescatterometer and/or the metrology tool.

SUMMARY

It is difficult to combine all the above requirements in a singleoptical system, however, a solution may be to simplify the optics andusing easily manufacturable optics of which the characteristics may bewell-known and using computational algorithms to improve images recordedby the simplified optical system. Exemplary methods and apparatusdisclosed herein relate to an architecture of a scatterometer and/ormetrology tool that has a simplified optical system that comprises areduced number of optical elements.

According to an aspect of the invention, there is provided a metrologytool for determining a parameter of interest of a structure fabricatedon a substrate, the metrology tool comprising: an illumination opticalsystem for illuminating the structure with illumination radiation undera non-zero angle of incidence; a detection optical system comprising adetection optical sensor and at least one lens for capturing a portionof illumination radiation scattered by the structure and transmittingthe captured radiation towards the detection optical sensor, wherein theillumination optical system and the detection optical system do notshare an optical element.

Optionally, at least part of an optical axis of the illumination opticalsystem is substantially parallel to an optical axis of the detectionoptical system.

Optionally, at least part of the illumination optical system ispositioned radially outwards from a radial extent of the detectionoptical system.

Optionally, the illumination optical system comprises a plurality ofdiscrete optical paths.

Optionally, at least two of the plurality of optical paths arediametrically opposed.

Optionally, the illumination optical system comprises at least onemirror for reflecting illumination radiation towards the structure.

Optionally, the metrology tool comprises at least one mirror in one ormore of the plurality of optical paths.

Optionally, the at least one mirror is configured to direct theillumination radiation onto the structure through a volume between atleast one lens of the detection optical system and the substrate.

Optionally, the at least one mirror is configured to direct radiationhaving a plurality of wavelengths in a range from 300 nm to 2 μm tosubstantially the same point on the substrate.

Optionally, the at least one mirror is one of an elliptical or aparabolic mirror.

Optionally, the at least one mirror has a reflectivity greater than 90%across a range of wavelengths of the illumination radiation.

Optionally, the detection optical system has a total transmissivity ofgreater than 90%.

Optionally, the detection optical system comprises 5 optical elements orfewer, for example, a single optical element, or two optical elements,or three optical element or four optical elements.

Optionally, the detection optical system comprises one or more of: aplano-convex lens; an aspheric lens; and a long working distanceobjective.

Optionally, the detection optical system comprises a plurality oflenses, and wherein one of the plurality of lenses positioned closest tothe substrate has a working distance between the substrate and a surfaceof the lens of one of: greater than 300 μm; greater than 500 μm; and ina range from 300 μm to 10 mm.

Optionally, the lens has a numerical aperture of: greater than 0.4;greater than 0.7; greater than 0.9; or 0.95 or greater.

Optionally, the metrology tool further comprises a focus systemcomprising at least one focussing optical sensor configured to receivezeroth order radiation reflected from the structure and a computationalimaging processor configured to determine a focus of the detectionoptical system based on.

Optionally, the focussing optical sensor comprises a quad opticalsensor, arranged such that a proportion of the reflected zeroth orderradiation that is sensed by each optical sensor in the quad opticalsensor is indicative of a position of the structure.

Optionally, at least one of the plurality of discrete optical paths isconfigured to receive, at least in part, reflected zeroth orderradiation originating from at least one other of the optical paths, andwherein the at least one of the plurality of discrete optical pathscomprises a reflective optical element configured to direct the receivedzeroth order radiation towards the at least one focussing opticalsensor.

Optionally, the at least one focussing optical sensor is positionedradially outwards from a radial extent of the detection optical system.

Optionally, the metrology tool further comprises a polarization elementarranged around an outer of the detection optical system and configuredto interact with radiation propagating through the illumination opticalsystem for polarization thereof.

Optionally, the polarization element is configurable to apply one of sor p polarization at a plurality of magnitudes.

Optionally, the polarization element is rotatable to apply one of s or ppolarization at one of a plurality of magnitudes.

Optionally, the metrology tool is configured to apply one of s or ppolarization at one of a plurality of magnitudes to one or more of theplurality of optical paths.

Optionally, the detection optical sensor is configured to acquire afirst image based on reflected and/or diffracted radiation having oneorder, and further configured to acquire a second image based onreflected and/or diffracted radiation having a further order.

Optionally, the metrology tool comprises a shutter system positioned inat least one of the plurality of optical paths configurable between anopen position in which illumination radiation is allowed to pass and aclosed position in which illumination radiation is blocked.

Optionally, the shutter system comprises one or moreacousto-optic-tunable filters.

Optionally, the metrology tool further comprises a reference opticalsensor, wherein the at least one of the plurality of optical pathscomprises a beam splitter configured to direct a proportion of theillumination radiation to the reference optical sensor when the shuttersystem is in the open position.

Optionally, the proportion of the illumination radiation is less than5%.

Optionally, the metrology tool further comprises an image normalisationunit configured to normalise the first and second images based on areference image acquired by the reference optical sensor.

Optionally, the metrology tool further comprises an acquisitioncontroller configured to control the reference optical sensor, thedetection optical sensor and the shutter system for capturing the firstand second images.

Optionally, a first shutter system is positioned in a first optical pathof the illumination optical system and a second shutter system ispositioned in a second optical path of the illumination optical system,and wherein the first shutter system is operable for acquiring the firstimage and the second shutter system is operable for acquiring the secondimage.

Optionally, the acquisition controller is configured to place thereference optical sensor in an acquisition phase and to open and closethe first and second shutter systems sequentially while the referenceoptical sensor is in the acquisition phase.

Optionally, the acquisition controller is further configured to placethe detection optical sensor in a first acquisition phase such that thefirst shutter system is opened and closed while the detection opticalsensor is in the first acquisition phase for acquiring the first image,and wherein the acquisition controller is further configured to placethe detection optical sensor in a second acquisition phase such that thesecond shutter system is opened and closed while the detection opticalsensor is in the second acquisition phase for acquiring the second image

Optionally, the illumination optical system and the detection opticalsystem has a footprint less than the area of a field of the substrate.The field size may have a dimension of 300 mm.

Optionally, the combination of the illumination optical system and thedetection optical system has at least one of an x-dimension and ay-dimension less than 26 mm.

Optionally, the metrology tool may comprise a plurality of illuminationoptical system and detection optical system combinations in an array.Each of the plurality of illumination optical system and detectionoptical system combinations may comprise an array element. Each arrayelement having a sensing axis.

Optionally, each array element is aligned with a different field of thesubstrate.

Optionally, the array elements are arranged in a two dimensional array.

Optionally, each array element has an array element footprint area onthe substrate and the array comprises a tessellation of footprint areas.

Optionally, the footprint areas within the array are the same and one oftriangular, square, rectangular or hexagonal in shape.

Optionally, the footprint areas are arranged in a honeycomb.

Optionally, the two dimensional comprises m rows and n columns, whereinm and n are both greater than 2.

Optionally, the array is adjustable such that the separation betweenadjacent array elements can be altered in at least one of the xdirection or y direction.

Optionally, the elements of the array sensors are tiltable with respectto the plane of the substrate, such that the sensing axis of each arrayelement is adjustable so as to be perpendicular with a substrate.

Optionally, each array element comprises a tilt sensor.

Optionally, the tilt sensor is located within the detection opticalsystem.

Optionally, the tilt sensor is an optical sensor.

Optionally, the sensing element is located within the detection opticalsensor.

Optionally, each array element is rotatable about the sensing axis.

Optionally, the element is configured to be rotatable about the sensingaxis from a first position to a second position, wherein the first andsecond positions are in anti-parallel.

Optionally, each array element comprises one or more actuatorsconfigured to move the array elements.

Optionally, the actuators comprise piezo motors.

Optionally, the metrology tool may further comprise: a controller,wherein the controller is configured to position the array elements at apredetermined pitch, wherein the predetermined pitch corresponds to thepitch of metrology targets on a substrate.

Optionally, the spacing of the array elements corresponds to a die pitchof a substrate.

Optionally, the pitch of the array elements in the x or y direction isone or more of 16.5 mm, 26 mm, 33 mm.

Optionally, the metrology tool may comprise: a coherent radiation sourceemitting illumination radiation that is received by a plurality ofoptical paths of the illumination optical system.

Optionally, the coherent radiation source comprises a laser emittingwhite light.

Disclosed herein is a metrology tool for determining a parameter ofinterest of a structure fabricated on a substrate, the metrology toolcomprising: an illumination optical system for illuminating thestructure with illumination radiation; an array of detection opticalsystems comprising a detection optical sensor and at least one lens forcapturing a portion of illumination radiation scattered by the structureand transmitting the captured radiation towards the detection opticalsensor, wherein each element of the array of detection optical systemsis adjustable such that the separation between adjacent elements of thearray can be altered in at least one of the x direction or y direction.

Also disclosed herein is a metrology tool for determining a parameter ofinterest of a structure fabricated on a substrate, the metrology toolcomprising: an illumination optical system for illuminating thestructure with illumination radiation; an array of detection opticalsystems comprising a detection optical sensor and at least one lens forcapturing a portion of illumination radiation scattered by the structureand transmitting the captured radiation towards the detection opticalsensor, wherein the array is a two dimensional array with array elementsarranged in a two dimensional array having m rows and n columns.

BRIEF DESCRIPTION OF DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying schematic drawings, in which:

FIG. 1 depicts a schematic overview of a lithographic apparatus;

FIG. 2 depicts a schematic overview of a lithographic cell;

FIG. 3 depicts a schematic representation of holistic lithography,representing a cooperation between three key technologies to optimizesemiconductor manufacturing;

FIG. 4 shows a schematic representation of an exemplary metrology tool;

FIGS. 5a and 5b show a schematic representation of an exemplarymetrology tool at different stages of operation

FIG. 6 shows a schematic representation of an exemplary metrology tool;

FIG. 7 shows a flow diagram for a method of operating a metrology tool;and

FIG. 8 shows a timing diagram corresponding to a method of operating ametrology tool.

FIGS. 9a and 9b show schematic representations of a further exemplarymetrology tool.

FIG. 10 shows a schematic representation of a further exemplarymetrology tool.

DETAILED DESCRIPTION

In the present document, the terms “radiation” and “beam” are used toencompass all types of electromagnetic radiation, including ultravioletradiation (e.g. with a wavelength of 365, 248, 193, 157 or 126 nm) andEUV (extreme ultra-violet radiation, e.g. having a wavelength in therange of about 5-100 nm).

The term “reticle”, “mask” or “patterning device” as employed in thistext may be broadly interpreted as referring to a generic patterningdevice that can be used to endow an incoming radiation beam with apatterned cross-section, corresponding to a pattern that is to becreated in a target portion of the substrate. The term “light valve” canalso be used in this context. Besides the classic mask (transmissive orreflective, binary, phase-shifting, hybrid, etc.), examples of othersuch patterning devices include a programmable mirror array and aprogrammable LCD array.

FIG. 1 schematically depicts a lithographic apparatus LA. Thelithographic apparatus LA includes an illumination system (also referredto as illuminator) IL configured to condition a radiation beam B (e.g.,UV radiation, DUV radiation or EUV radiation), a mask support (e.g., amask table) MT constructed to support a patterning device (e.g., a mask)MA and connected to a first positioner PM configured to accuratelyposition the patterning device MA in accordance with certain parameters,a substrate support (e.g., a wafer table) WT constructed to hold asubstrate (e.g., a resist coated wafer) W and connected to a secondpositioner PW configured to accurately position the substrate support inaccordance with certain parameters, and a projection system (e.g., arefractive projection lens system) PS configured to project a patternimparted to the radiation beam B by patterning device MA onto a targetportion C (e.g., comprising one or more dies) of the substrate W.

In operation, the illumination system IL receives a radiation beam froma radiation source SO, e.g. via a beam delivery system BD. Theillumination system IL may include various types of optical components,such as refractive, reflective, magnetic, electromagnetic,electrostatic, and/or other types of optical components, or anycombination thereof, for directing, shaping, and/or controllingradiation. The illuminator IL may be used to condition the radiationbeam B to have a desired spatial and angular intensity distribution inits cross section at a plane of the patterning device MA.

The term “projection system” PS used herein should be broadlyinterpreted as encompassing various types of projection system,including refractive, reflective, catadioptric, anamorphic, magnetic,electromagnetic and/or electrostatic optical systems, or any combinationthereof, as appropriate for the exposure radiation being used, and/orfor other factors such as the use of an immersion liquid or the use of avacuum. Any use of the term “projection lens” herein may be consideredas synonymous with the more general term “projection system” PS.

The lithographic apparatus LA may be of a type wherein at least aportion of the substrate may be covered by a liquid having a relativelyhigh refractive index, e.g., water, so as to fill a space between theprojection system PS and the substrate W—which is also referred to asimmersion lithography. More information on immersion techniques is givenin U.S. Pat. No. 6,952,253, which is incorporated herein by reference.

The lithographic apparatus LA may also be of a type having two or moresubstrate supports WT (also named “dual stage”). In such “multiplestage” machine, the substrate supports WT may be used in parallel,and/or steps in preparation of a subsequent exposure of the substrate Wmay be carried out on the substrate W located on one of the substratesupport WT while another substrate W on the other substrate support WTis being used for exposing a pattern on the other substrate W.

In addition to the substrate support WT, the lithographic apparatus LAmay comprise a measurement stage. The measurement stage is arranged tohold a sensor and/or a cleaning device. The sensor may be arranged tomeasure a property of the projection system PS or a property of theradiation beam B. The measurement stage may hold multiple sensors. Thecleaning device may be arranged to clean part of the lithographicapparatus, for example a part of the projection system PS or a part of asystem that provides the immersion liquid. The measurement stage maymove beneath the projection system PS when the substrate support WT isaway from the projection system PS.

In operation, the radiation beam B is incident on the patterning device,e.g. mask, MA which is held on the mask support MT, and is patterned bythe pattern (design layout) present on patterning device MA. Havingtraversed the mask MA, the radiation beam B passes through theprojection system PS, which focuses the beam onto a target portion C ofthe substrate W. With the aid of the second positioner PW and a positionmeasurement system IF, the substrate support WT can be moved accurately,e.g., so as to position different target portions C in the path of theradiation beam B at a focused and aligned position. Similarly, the firstpositioner PM and possibly another position sensor (which is notexplicitly depicted in FIG. 1) may be used to accurately position thepatterning device MA with respect to the path of the radiation beam B.Patterning device MA and substrate W may be aligned using mask alignmentmarks M1, M2 and substrate alignment marks P1, P2. Although thesubstrate alignment marks P1, P2 as illustrated occupy dedicated targetportions, they may be located in spaces between target portions.Substrate alignment marks P1, P2 are known as scribe-lane alignmentmarks when these are located between the target portions C.

As shown in FIG. 2 the lithographic apparatus LA may form part of alithographic cell LC, also sometimes referred to as a lithocell or(litho)cluster, which often also includes apparatus to perform pre- andpost-exposure processes on a substrate W. Conventionally these includespin coaters SC to deposit resist layers, developers DE to developexposed resist, chill plates CH and bake plates BK, e.g. forconditioning the temperature of substrates W e.g. for conditioningsolvents in the resist layers. A substrate handler, or robot, RO picksup substrates W from input/output ports I/O1, I/O2, moves them betweenthe different process apparatus and delivers the substrates W to theloading bay LB of the lithographic apparatus LA. The devices in thelithocell, which are often also collectively referred to as the track,are typically under the control of a track control unit TCU that initself may be controlled by a supervisory control system SCS, which mayalso control the lithographic apparatus LA, e.g. via lithography controlunit LACU.

In order for the substrates W exposed by the lithographic apparatus LAto be exposed correctly and consistently, it is desirable to inspectsubstrates to measure properties of patterned structures, such asoverlay errors between subsequent layers, line thicknesses, criticaldimensions (CD), etc. For this purpose, inspection tools (not shown) maybe included in the lithocell LC. If errors are detected, adjustments,for example, may be made to exposures of subsequent substrates or toother processing steps that are to be performed on the substrates W,especially if the inspection is done before other substrates W of thesame batch or lot are still to be exposed or processed.

An inspection apparatus, which may also be referred to as a metrologytool, is used to determine properties of the substrates W, and inparticular, how properties of different substrates W vary or howproperties associated with different layers of the same substrate W varyfrom layer to layer. The inspection apparatus may alternatively beconstructed to identify defects on the substrate W and may, for example,be part of the lithocell LC, or may be integrated into the lithographicapparatus LA, or may even be a stand-alone device. The inspectionapparatus may measure the properties on a latent image (image in aresist layer after the exposure), or on a semi-latent image (image in aresist layer after a post-exposure bake step PEB), or on a developedresist image (in which the exposed or unexposed parts of the resist havebeen removed), or even on an etched image (after a pattern transfer stepsuch as etching).

Typically the patterning process in a lithographic apparatus LA is oneof the most critical steps in the processing which requires highaccuracy of dimensioning and placement of structures on the substrate W.To ensure this high accuracy, three systems may be combined in a socalled “holistic” control environment as schematically depicted in FIG.3. One of these systems is the lithographic apparatus LA which is(virtually) connected to a metrology tool MT (a second system) and to acomputer system CL (a third system). The key of such “holistic”environment is to optimize the cooperation between these three systemsto enhance the overall process window and provide tight control loops toensure that the patterning performed by the lithographic apparatus LAstays within a process window. The process window defines a range ofprocess parameters (e.g. dose, focus, overlay) within which a specificmanufacturing process yields a defined result (e.g. a functionalsemiconductor device)—typically within which the process parameters inthe lithographic process or patterning process are allowed to vary.

The computer system CL may use (part of) the design layout to bepatterned to predict which resolution enhancement techniques to use andto perform computational lithography simulations and calculations todetermine which mask layout and lithographic apparatus settings achievethe largest overall process window of the patterning process (depictedin FIG. 3 by the double arrow in the first scale SC1). Typically, theresolution enhancement techniques are arranged to match the patterningpossibilities of the lithographic apparatus LA. The computer system CLmay also be used to detect where within the process window thelithographic apparatus LA is currently operating (e.g. using input fromthe metrology tool MT) to predict whether defects may be present due toe.g. sub-optimal processing (depicted in FIG. 3 by the arrow pointing“0” in the second scale SC2).

The metrology tool MT may provide input to the computer system CL toenable accurate simulations and predictions, and may provide feedback tothe lithographic apparatus LA to identify possible drifts, e.g. in acalibration status of the lithographic apparatus LA (depicted in FIG. 3by the multiple arrows in the third scale SC3).

FIG. 4 shows an exemplary metrology tool 100. In metrology tool 100there is a detection optical system that comprises two lenses 110, 150.In another embodiment only lens 150 is present in the detection opticalsystem. In another embodiment, more lenses may be present in thedetection optical system. In an embodiment the lenses 110, 150 arecapable of transmitting and refracting radiation in a wide wavelengthrange, e.g. from 200 nm to 2000 nm, from 200 nm to 1000 nm or from 400nm to 800 nm. The lenses 110, 150 may have a relatively large aberrationand the aberration of the lenses 110, 150 may be known and is e.g.stored as a known aberration, KNWL ABR, 132.

Lens 150 captures a portion of radiation that is scattered by astructure 128 fabricated on, for example, a semiconductor substrateand/or a wafer. A working distance of the lens 150 (e.g. the distancebetween a surface of the lens 150 and the structure 128) may be greaterthan 300 μm, greater than 500 μm or in a range from 300 μm to 1 mm. Thestructure 128 may be a product structure or may be a specificallydesigned metrology structure, a so-termed (metrology) target, to measurecertain characteristics of the process that is being applied to thesemiconductor substrate and/or wafer. Captured radiation 106 maysubsequently be imaged by lens 110 on a detection optical sensor 102,which may be a sensor with a pixel array that registers per pixel anamount of impinging radiation.

The sensor 102 may be a single sensor and may be capable of detectingradiation in a wavelength range from 200 nm to 2000 nm, and, in anotherembodiment, the sensor 102 is capable of detecting a smaller wavelengthrange, for example, from 200 nm to 1000 nm or from 400 nm to 800 nm. Inanother embodiment, the captured beam of radiation 106 is split into twoseparate beams and both of the separate beams are imaged on a separatesensor that each are sensitive to another wavelength range. The lenses110, 150, and optionally also the sensor 102, may be provided in a tube104. The lens 150 may have a relatively small NA, for example, smallerthan 0.4, and in other embodiment the NA of lens 150 is larger, forexample greater than 0.4, greater than 0.7, greater than 0.9 or 0.95 orgreater. Lens 150 can be of any type of lens and may have asphericsurfaces designed for minimizing aberrations. The lens 150 may be one ofa plano-convex lens; an aspheric lens; and a long working distanceobjective.

An illumination optical system for illuminating the structure 128 isseparated from the detection optical system and shares as few aspossible optical elements with the detection optical system. In onespecific example, the illumination optical system and the detectionoptical system do not share an optical element. In exemplaryarrangements, the illumination optical system may comprise two, four ormore discrete optical paths and at least two of the discrete opticalpaths may be diametrically opposed. As used herein, an “optical element”encompasses elements that reflect or refract radiation, such as, forexample mirrors and lenses.

Using the arrangement disclosed in FIG. 4, illumination radiation may beprovided by one or both of the fibers 112, 140. The fibers 112, 140 maybe single mode fibers and this may optionally result in single modeillumination of the structure on the substrate. The radiation 124 thatis output from the fibers 112, 140 falls in the presented embodiment onmirror 126 or mirror 148, respectively, which subsequently reflects theradiation towards the structure 128. In the arrangement of FIG. 4, theillumination radiation is directed onto the structure 128 through avolume between the substrate and the lens 150 of the detection opticalpath. The mirrors 126, 148 may be elliptically shaped mirrors. Theelliptically shaped mirrors may image the tips of the fibers 112, 140 onthe structure 128. In another embodiment, the mirrors 126, 148 areparabolic mirrors. The mirrors 126, 148 are configured to directradiation having a plurality of wavelengths in a range from 300 nm to 2μm, 300 nm to 1 μm or 400 nm to 800 nm to substantially the same pointon the substrate to form an illumination spot. The reflectivity of themirrors 126, 148 may in some embodiments be substantially uniform forany of the above wavelength ranges.

Assuming that only fiber 112 provides illumination radiation, then thestructure 128 reflects a portion of the illumination radiation towardsmirror 148 and a portion of the radiation that is scattered by thestructure 128 is captured by lens 150. In an embodiment radiation in oneor more diffraction orders is captured by the lens 150.

It is also possible to use only fiber 140 to provide illuminationradiation and then structure 128 reflects a portion of the illuminationradiation towards mirror 126 and scattered light, for example of one ormore diffraction orders opposite the diffraction orders of radiationfrom mirror 124 and diffracted by the structure 128, is captured by lens150.

The illumination optical system may also comprise two beam splitters114, 142 in between the tip of the fibers 112, 140 and mirrors 126, 148.Light that is reflected by the structure 128 and that falls on themirrors 126, 148 is at least partially reflected towards a focus systemcomprising focussing optical sensors 120, 144 which have multiple areasthat detect per area how much radiation impinges on the respectiveareas. For example, the focussing optical sensors 120, 144 have fourquadrants wherein e.g. each quadrant comprises a photodiode capable ofdetecting the intensity of light that impinges on the quadrant. Theposition of the focussing optical sensors 120, 144 may be chosen suchthat the reflected illumination radiation forms a spot on the focussingoptical sensor 120, 144. The four quadrants are for example shown at 116together with such a spot 118.

The data recorded by the focussing optical sensors 120, 144 can be usedin the focus system. If the structure 128 moves in the z-direction (upand down in FIG. 4) then the position of the spot moves over thefocussing optical sensor 120 or focussing optical sensor 144 and at acertain position the structure 128 is at an optimum focus of theillumination radiation. By using the differently recorded intensities inthe four quadrants, it can be determined at which position the spot 118is on the focussing optical sensor 120 (or focussing optical sensor144). A calibration mechanism can be used to determine the position atwhich the spot must be at the optimum focus position of the structure128.

It is to be noted that, in certain embodiments of the metrology system100, it is not necessary that the structure 128 is always exactly inoptimum focus. Also if radiation is scattered at an out of focusposition and if this scattered radiation is detected by sensor 102, thena processing unit, such as a computational imaging processor, COMPU IM,136, algorithm may be capable of creating an in-focus image. Forexample, this may be enabled if information is available that comprisesvalues that relate to how much is the structure 128 out of focus. Forexample, the data detected by the focussing optical sensors 120, 144 canbe used in such a computational imaging, COMPU IM, 136, algorithm.Scattered radiation may be captured by the sensor 102 in two images eachhaving different focus levels in order to retrieve phase information.

The design of the detection optical system and the illumination opticalsystem is such that the combination of both is relatively compact. Theillumination optical system, with its different sub-systems, arearranged directly around lens 150 and do not require much space or largeoptical elements. For example, a cross-sectional diameter (i.e., width“ω”) of the combination of the detection optical system and theillumination optical system may be smaller than 50 mm, or even smallerthan 30 mm. This design allows the use of several instances of metrologytool 100 close to each other such that different structures, such asstructure 128, can be inspected or measured simultaneously.

At least part of the illumination optical system and the detectionoptical system are parallel to each other. That is, the illuminationoptical beam and the detection optical beam are parallel to each otherduring at least part of the respective paths. The at least part of therespective paths may also be transverse and optionally at right anglesto the substrate comprising the structure 128. More specifically and asshown in FIG. 4, at least part of the illumination optical system isradially outwards from the radial extent of the detection opticalsystem, in this case defined by the tube 104.

The illumination optical system and the detection optical systemtogether may have a footprint less than the area of a field of thesubstrate. In some examples, the combination of the illumination opticalsystem and the detection optical system has at least one of anx-dimension and a y-dimension less than 26 mm. In this context, thex-dimension and y-dimension are those that are transverse to thez-dimension (described above and vertical in FIG. 4).

Exemplary metrology tools may comprise a plurality of illuminationoptical system and detection optical system combinations according toany disclosed herein. Each of the plurality of illumination opticalsystem and detection optical system combinations may be configured suchthat they are aligned with a different field of the substrate.

The fibers 112, 140 may receive radiation from, for example, a laser.The laser may be capable of generating radiation in a relatively widewavelength range, for example, from 200 nm to 2000 nm. The laser may bea tunable laser which generates a single wavelength and is tunabletowards such a single wavelength in a relatively wide wavelength range.The laser may also be capable of generating radiation along the wholeabove mentioned wavelength range and filters may be used to select onlya portion or only a single wavelength from this relatively widewavelength range.

Exemplary metrology tools 100 may have a polarization element, which inthis case is a polarizing ring 122 of which a top view 146 is presentedin FIG. 4. The ring may be arranged to be rotatable around the tube 104and lens 150 and may be arranged at such a position that illuminationradiation that travels from the tip of one of the fibers 112, 140towards its respective mirror 126, 148 via a portion of the polarizingring 122. The polarizing ring 122 may have sections that polarize theradiation that travels through it into P or S polarized radiation. Thepolarizing ring 122 may have 8 sections that may be equally arrangedalong its circumference. Different types of polarizing areas alternatein each section. This allows the illumination of the structure 128 withillumination that has a certain polarization.

In some exemplary arrangements, the polarization element may have thesame polarization for X (or Y) gratings; independent polarizationbetween X and Y gratings; and/or support any polarization mode.Providing this with one disk may utilize either a disk with 36polarization holes or a mechanism for continuous movement of the diskbetween X and Y gratings.

An alternative embodiment may comprise two disks which can rotateindependently from each other. One addresses the X (and has an open slotfor Y) and vice-versa for the other disk.

At the right top of FIG. 4, a possible workflow of the metrology tool ispresented. For example, sensor 102 may provide a dark-field image 130which is subject to aberrations of the lenses 150, 110 and which mayprovide an image at a non-ideal focus position of structure 128. Thisimage 130 is input to a computational imaging processor, COMP IM, 136running a computational imaging algorithm. The computational imagingprocessor, COMP IM, 136, may also receive multiple images 130, forexample, recorded at different non-ideal focus positions. Together withthe recorded images 130, the computational imaging processor, COMP IM,136, may receive focus related information from one or more opticalsensors 120, 144, and the computational imaging processor, COMP IM, 136,may receive knowledge about the aberrations of the lenses, KNWL ABR,132. This information may be used by the computational imagingprocessor, COMP IM, 136, to obtain a better dark field image 134 and/orto obtain a complex field of radiation scattered by the structure 128.

Although in the above figure only two fibers 112, 140, two beamsplitters 114, 142, two mirrors 126, 148 and two focussing opticalsensors 120, 144 are presented, more of these elements may be providedaround the tube 104. For example, in a top view of metrology tool 100,there may be four illumination radiation sub-systems arranged aroundtube 104 and lens 150. One or more of the four illumination radiationsub-systems may comprise at least one of a fiber, a beam splitter, amirror and/or an optical sensor. Seen in the top view, theseillumination radiation sub-systems may be provided at 0, 90, 180 and 270degrees around the tube 104. Such a configuration allows the measurementof structure 128 from two orthogonal directions (e.g. x and y) andallows the detection of diffraction orders and opposite diffractionorders per one of the orthogonal directions without rotating structure128 with respect to lens 150.

In exemplary arrangement of FIG. 4, the radiation that is reflected bythe structure, in other words, the zeroth diffraction order, is not lostand is at least partially propagated toward the focussing opticalsensors 120, 144. In an embodiment, these sensors can also be used todetect the intensity of the zeroth diffraction order and thereby thisinformation can also be used to determine certain parameters of thestructure 128. This allows a zeroth order spectral measurement.

If the intensity of the zeroth diffraction order is measured, and byusing the above discussed polarizing ring 122 in a specific manner or byusing a modified design of the polarizing ring, one can enable zerothorder cross-polarized measurements. E.g. the polarizing ring 122 can bemade to have crossed polarizers at the diametrically opposite parts.

FIGS. 5a and 5b show simplified schematic representations of a metrologytool 200. One or more of the features of the metrology tool 100described with respect to FIG. 4 may also be present in the metrologytool 200 as appropriate and these are not described again here indetail.

The metrology tool 200 comprises a shutter in at least one of theoptical paths of the illumination optical system. In the example ofFIGS. 5a and 5b , there are two optical paths and each optical pathcomprises a shutter 202 a, 202 b. In known exemplary arrangements,mechanical shutters are used. Such shutters typically have a switchingspeed (i.e. the time required to open or close the shutter) in the orderof milliseconds.

The metrology tool 200 also comprises an optical sensor 204. The opticalsensor 204 is positioned in the detection optical system and maytherefore be termed a detection optical sensor. In some arrangements,the metrology tool 200 may also comprise features similar to or the sameas the focussing optical sensors 120, 144. In exemplary arrangements,the optical sensor 204 may be a high-speed CMOS camera with at least1000 frames per second.

As shown in FIGS. 5a and 5b , the metrology tool 200 may operate bysequential opening and closing of the shutters 202 a, 202 b to directillumination radiation from alternate directions onto the structure 206,which diffracts it through the lens 208 and onto the optical sensor 204.This allows sequential measurement images to be captured, a measurementimage being one formed on the detection optical sensor 204. FIG. 5arepresents the configuration capturing the first measurement image andFIG. 5b represents the configuration capturing the second measurementimage some time later. In exemplary arrangements, the illuminatingradiation through a first shutter 202 a provides the first measurementimage comprising +1^(st) order diffracted radiation, and theilluminating radiation through a second shutter 202 b provides thesecond measurement image comprising −1^(st) order diffracted radiation.The sequential images may be used by a processing unit, such as thecomputational imaging processor, COMP IM, 136 discussed above todetermine a parameter of the structure 206.

In an ideal scenario, to use the sequential images to determine aparameter of interest of the structure 206, such as overlay, theintegrated radiation intensity of each of the sequential measurementimages would be the same. In practical implementations, the integratedradiation intensity of the sequential measurement images should bewithin 0.01% of each other. However, operation of the shutters 202 a,202 b and acquisition of the sequential measurement images by theoptical sensor 204 introduce errors in the shape of jitter.Specifically, acquisition time jitter is a random variation in the startand end of an acquisition time for each of the sequential measurementimages, which affects the integrated radiation intensity of eachmeasurement image. The acquisition time jitter manifests as a randomvariation in the number of collected photons in each pixel of an imagesince this scales linearly with acquisition time. In the end thisresults in a random error in the measured intensity difference betweenthe −1^(st) and +1^(st) orders which results in noise in measurement ofthe parameter of the structure 206, such as overlay. Other error sourcesalso may affect the integrated radiation intensity of each measurementimage, such as the different brightness of each radiation source 210 a,210 b.

FIG. 6 shows a further exemplary metrology tool 300. The metrology tool300 may comprise one or more of the features of FIGS. 5a and 5b , asappropriate, and those features are not discussed again here.

In addition, metrology tool 300 comprises a reference optical sensor 350that is configured to capture a reference image having an acquisitiontime equal to that of the first and second sequential images discussedabove in respect of FIGS. 5a and 5b . That is, the reference image issubject to the same acquisition time jitter as each of the first andsecond images. The reference image may be used to normalise the firstand second measurement images and thereby mitigate or remove errorsassociated with acquisition time jitter and/or intensity of radiationsource.

The metrology tool 300 comprises a single radiation source 352, whichmay be a supercontinuum source. The radiation source 352 is configuredto emit radiation in a source beam towards a source beam splitter 354.The source beam splitter 354 is configured to split the source beam intotwo beams and direct a first beam towards a first shutter 302 a and asecond beam towards a second shutter 302 b. In a specific arrangement,the first beam and the second beam have substantially equal radiationintensities.

In the exemplary arrangement of FIG. 6, the shutters 302 a, 302 bcomprise acousto-optic-tunable filters (AOTFs) that are operated by anacoustic signal emitted by first and second acoustic signal generators356 a, 356 b. Using AOTFs as shutters has the advantage that theshutters 302 a, 302 b are able to open and close more quickly. If theshutters 302 a, 302 b are open then the first and second beamsrespectively are propagated towards first and second reference beamsplitters 358 a, 358 b. The reference beam splitters 358 a, 358 b areconfigured to split each of the first and second beams respectively intoa first reference beam 360 a, a first measurement beam 362 a, a secondreference beam 360 b and a second measurement beam 362 b. In exemplaryarrangements, the first and/or second reference beams 360 a, 360 b havea radiation intensity that is less than 20%, less than 10%, less than 5%or may specifically be 5% or 1% of the first or second beamrespectively.

An acquisition controller 364, which may for part of the processingunit, COMP IM, 136, or may be a separate processing unit is configuredto control the reference optical sensor 350 and the detection opticalsensor 304 to capture the reference image and the measurement image. Theacquisition controller may also control the shutters 302 a, 302 b viathe acoustic signal generators 356 a, 356 b to ensure that acquisitionof the images is within an open time of each shutter, accounting forshutter jitter. Operation of the acquisition controller 364 is discussedin detail below with reference to FIGS. 7 and 8.

In the particular example of FIG. 6, the measurement images are acquiredat a framerate of 1 kHz while the reference images are acquired at halfthat framerate (i.e. 500 Hz).

Referring to FIGS. 7 and 8, a method of operating a metrology tool suchas those disclosed herein is described.

The radiation source 352 generates 700 radiation having a range ofwavelengths such as those described herein. The source beam splitter 354splits 702 the source beam into two beams that, in one exemplaryarrangement comprise equal proportions of the source beam. A first beamis received by a first shutter 302 a and a second beam is received by asecond shutter 302 b. The first and second shutters 302 a, 302 b areconfigured to block or allow the first and second beams to pass underthe control of first and second acoustic signal generators 356 a, 356 b.

The acquisition controller 364 controls 704 the reference optical sensor350 to begin the acquisition time 400 of the reference images, andcontrols the detection optical sensor 304 to begin the acquisition time402 of the first measurement image. At this stage, the shutters 302 a,302 b are closed. After the acquisition times of the reference and firstmeasurement images has begun, accounting for any jitter 404, 406, theacoustic signal generators 356 a, 356 b are controlled to opensequentially the first and second shutters 302 a, 302 b.

In exemplary arrangements of FIG. 6, a wavelength measuring unit may beadded, which controls the output of the AOTFs 302 a, 302 b. Analternative embodiment may comprise one AOTF for both the −1^(st) orderand +1^(st) order optical paths. In that case the wavelength deltabetween these two paths would be negligible and it would save one AOTF,in which case the AOTF may be positioned before the beamsplitter 354.

Further, an AOTF typically has polarized light as its output and,accordingly, exemplary arrangements may comprise a depolarizer after theAOTF.

Given the needed measurement accuracy between −1^(st) order and +1^(st)order images, exemplary arrangements may also comprise a feature forpreventing light coming from one side going into the other side via thetarget and create a signal on the image sensor. One option may be todeliberately put the two optical paths not in line with each other.Alternatively, one could use two images.

As shown in FIGS. 7 and 8, the first shutter 302 a is opened 706, 408while the second shutter 302 b remains closed. This allows the firstbeam to propagate to the reference beam splitter 358 a, which directs aportion of the first beam to the reference optical sensor 350 and allowsthe remainder of the first beam to be directed to the structure 306 andon to the detection optical sensor 304 for measurement of a parameter ofthe structure as disclosed above. Sometime later, the first shutter 302a is closed 410. The first reference and measurement images are therebycaptured. After capturing the first reference and measurement images,the acquisition controller 364 may end 412 the acquisition time of thefirst measurement image.

Because the acquisition of the first reference and measurement images iscontrolled by the first shutter 302 a, the acquisition time of each isthe same. Therefore, the integrated radiation intensity of the firstreference image is proportional to that of the first measurement imageand so can be used to normalise the first measurement image and removethe effects of jitter. The +1 order image and −1 order image arereferred as I₊₁ and L⁻¹. The reference images are referred to asI__(ref_+1) and I__(ref_−1). A region selection of the reference imagesis made from where to calculate the mean reference intensity. Theselection may be of the full frame or a specific set of pixels. Thisregion is called ROI. The mean reference values are calculated as themean of the pixel values of the selected pixels and is referred asI_(μ_ref_+1) and I_(μ_ref_−1). The normalized intensities used for theoverlay calculations are calculated as I_(+1_norm)=I₊₁/I_(μ_ref_+1) andI_(−1_norm)=I⁻¹/I_(μ_ref_−1). Where the / sign refers to a division ofthe all the pixel values of the +1 and −1 order image with the meanreference value

The process is repeated for the second shutter 302 b, in that theacquisition time of the second measurement image is begun 414, the firstshutter 302 a is closed, the second shutter 302 b is opened 708, 416 andsubsequently closed 418 to capture the second reference and measurementimages. The acquisition controller 364 then ends the acquisition timesof the second measurement image 420 and the reference images 422.

The first and second measurement images are then normalised 710. Asmentioned above, the processing unit, COMP IM, 136 may be configured toprocess the first and second measurement images captured by thedetection optical sensor 304 based on first and second reference imagescaptured by the reference optical sensor 350. The processing unit, COMPIM, 136 may comprise a normalisation unit configured to normalise thefirst and second images accordingly.

In some arrangements, advantages may be provided by using optics thatallow the use of broadband light (e.g. 300 nm-2000 nm), which lowers thecosts of the metrology tools, allows a high throughput (which means thatthey have a high photon transmission efficiency) and/or allows ameasurement of locations on the wafer with e.g. multiple wavelengths (inparallel or very fast in a serial manner).

In some arrangements there may be a desire to increase the rate at whichmetrology can be acquired to minimise the time spent on metrology. Inorder to achieve this disclosed herein is a metrology tool which cancarry out metrology on multiple targets in parallel.

FIGS. 9a and 9b respectively show plan and side view schematicrepresentations of metrology tool arrangement 900 comprising an array ofdetection optical systems. Each of the detection optical systems maycomprise a detection optical sensor 910 and at least one lens forcapturing a portion of illumination radiation scattered by the structureand transmitting the captured radiation towards the detection opticalsensor 910, as described above in relation to FIGS. 4 and 6, forexample. As such, the detection optical system may form part of ametrology tool for determining a parameter of interest of a structurefabricated on a substrate, the metrology tool additionally comprising:an illumination optical system for illuminating the structure withillumination radiation. Hence, each of array elements 904 shown in FIG.9 may represent the system shown on the left hand side of FIG. 4, shownin FIG. 5a , shown in FIG. 5b and/or comprise a detection sensor withoutthe illumination optical system. For the purposes of the followingdescription, each of the optical detection systems will be referred toas elements of the array, or array elements 904.

Each of the array elements 904 includes an optical detector sensor 910which may, for example, be similar to that shown and described inrelation to FIG. 4, in relation to FIG. 5a , in relation to FIG. 5band/or in relation to FIG. 6. The optical detector sensor has a sensoraxis 906 which extends orthogonally between the detection optical sensor910 and plane of the substrate 902.

The substrate 902 includes a plurality of dies 908, as indicated by thelines shown in FIG. 9a . Each of the array elements 904 may bepositioned in relation to an individual die 908 and take metrology datatherefrom. Hence, each of a plurality of dies 908 may include a singleelement array 904. The array elements 904 may be located in a commonhorizontal location with respect to each of the dies 908, such thatmetrology markers which are common to each of the dies and havecorresponding positions in the dies can be measured in parallel by eachof the sensors 910.

The array may comprise a tiled or tessellated arrangement of opticaldetection systems. Each optical detection system may be provided in afootprint area having a predetermined shape in which the footprint areasabut one another to provide the array. Each footprint area may be thesame and may be polygonal, for example, each footprint area may betriangular, square or hexagonal. In one example, the array may beprovided as a tessellation of hexagonal footprint areas to provide ahoneycomb array. Thus, the array elements may be arranged in a honeycombarray. The array of optical detection systems is shown as a twodimensional array having m rows and n columns. The rows m may extend afirst direction, for example, the x direction of the substrate and thecolumns may extend in a second direction, for example, the y direction.Either or both of the directions of the rows and columns may be inclinedto the x or y direction. In general, the number of rows and columns willeach be greater than 2. However, there can be as many rows or columns asdesired for a particular field layout on a wafer or metrology footprint.The array may also be any desired shape and is not restricted to being asquare or rectangular configuration. The number of array elements 904can be varied to accommodate different applications. The size of thearray may be, for example, up to 15 in the x direction and up to 100 inthe y direction. In some examples, a single array element 904 may beprovided for each of the respective dies 908 so as to cover the entirewafer. In other applications a discrete number of array elements 904 maybe provided which is less than the number of dies 908. Each member ofthe array elements may be aligned with a different field of thesubstrate.

As indicated by the arrows 912 a, 912 b, 912 c the array may beadjustable such that the array elements 904 (or parts thereof) ismovable in relation to one another or the substrate 902. As shown,adjacent array elements 904 are separated from one another by a firstdistance. The first distance between adjacent array elements maycorrespond to the pitch of the dies or metrology targets. The firstdistance may be the same for all of the respective adjacent pairs of thearray elements 904. As shown by arrow 912 a, each of the array elements904 may be moved such that the separation between the adjacent arrayelements may be altered from the first distance to the second distance.The movement of the array elements 904 may be done in the x and/or ydirection.

The spacing may be a fixed pitch in accordance with the pitch of thedies 908 or metrology targets which are distributed across the substrate902. As such, the spacing of the array elements 904 may be standard andone of a number of predetermined discrete spacing setting whichcorrespond to standard features, such as the pitch of the die. Thepitches may be, for example, 26 mm in the x direction and 33 mm in the ydirection to correspond to conventional die sizes. Other pitches mayinclude 26 mm by 16.5 mm. The metrology tool 900 may incorporate or haveaccess to a list or library of one or more spacings which areconventional or frequently used. The predetermined spacings may formpart of a fabrication recipe or be included as part of a set-up processfor a particular process.

In order to obtain good metrology data, it is preferable that the sensoraxis 906 be aligned so as to be perpendicular to the plane of thesubstrate 902 surface. A substrate surface of a processed wafer 902 canshow local tilt variations of the order of several 100 microradians. Inorder to deal with these local tilt variations the array elements may betiltable 912 b so as to alter the angle between the sensor axis 906 andthe substrate 902 surface. The tilt may be restricted to two orthogonaldirections, for example, x and y directions where tipping the sensor inthe x direction would cause the sensor axis to move along the xdirection, and tipping the sensor in the y direction would cause thesensor axis to move along the y direction. Combinations of the twotipping directions may allow for any substrate tilt to be accommodated.As the variations in surface level may be local, each of the arrayelements may be moved independent from the other array elements 904.Typically, when tilting around the x-axis, motion in y direction willoccur (and the other way around)

In order to determine the tilt of the array element 904, a tilt sensormay be incorporated within each of the array elements. The tilt sensormay be an optical sensor as known in the art, and may advantageously beincorporated into the optical sensor 910. Thus, each of the arrayelements 904 may comprise a combined overlay and tilt sensor.

When the metrology tool is being used to obtain overlay metrology data,it is advantageous to be able to correct for measurement errors whichresult from sensor asymmetry. This error may be referred to asTool-Induced-Shift, TIS. To address TIS (or other similar issues), thearray elements 904 may each be rotatable about the sensing axis 906. Assuch, each of the array elements 904 may be rotated from a firstrotational position to a second rotational position in which the firstand second positions are antiparallel to the extent they need to be toaccount for tool induced shift. Thus, the sensors may be rotated throughapproximately 180 degrees. The rotation will typically be around theinsertion axis 906, however, a positional difference between the firstand second rotational positions with respect to the surface of thesubstrate can be accounted for when processing the acquired metrologydata. That is, a shift in the x-y position of the sensing axis 906 whichoccurs as a result of the rotation may be accommodated by modifying theoverlay data with alignment data taken from the respective first andsecond rotational positions.

The movement of the array elements 904 may be achieved using suitableactuators known in the art. The actuators may be, for example, piezomotors. Thus, each of the array elements 904 may include one moreactuators for each of the described ranges of motion.

The metrology tool may incorporate a positional controller 914 to whichis configured to control the movement of the array elements 904. As suchthe positional controller 914 will be in communication with each of theactuators so as to provide the necessary control signals. The positionalcontroller 914 may also be arranged to receive positional data eitherfrom the actuators, array elements 904 or some other source which canprovide an indication of position of the array elements relative to thesubstrate 902. The positional controller 914 may be distributed amongthe element arrays or be provided as a central unit which is arranged tocontrol all of the array elements individually. The central unit may belocal to the metrology tool 900 or located remotely. The positionalcontroller 914 may form part of a larger control system such as thecomputer system CL described above.

In use, the positional controller 914 will receive or determine adesired spacing between the adjacent array elements before moving eachof the array elements 904 into the correct position.

Once the array elements 904 have been positioned, the position verifiedand any adjustments made, metrology data can the obtained as describedabove.

The array elements 904 may be separately controllable. The arrayelements 904 may have one or more of the above described ranges ofmovement. Hence, there may be examples in which the array elements 904are arranged to tilt but not move in the x-y directions. This may beuseful where the spacing of the array elements can be fixed in the x-ydirections.

The array element 904 may be similar or equal array elements 904. Forexample, all array elements may be configured to operate in the sameoperational wavelength range. Even if the array element 904 are similaror equal, this does not exclude that each array element may receiveillumination radiation of another wavelength in the full operationalwavelength range of the array element, e.g. different array elementsreceive light at different wavelengths in the operational wavelengthrange of 200 to 2000 nm

However, it is not necessary that all array elements 904 are similar orequal to each other. There may be at least one array element 904 that isdifferent from the other array elements 904. The array element 904 mayalso be subdivided in groups of array elements 904 and within each groupof array elements 904 the array element are similar or equal, but theymay differ over the groups of array elements 904. For example, a firstrow or column of the array comprises a first type of array element, thesecond row or column of the array comprises a second type of arrayelements, etc. It is to be noted that one array element comprises acombination of an illumination optical system and a detection opticalsystem. If array elements are different, their respective illuminationoptical systems and/or their respective detection optical systems maydiffer from each other.

A difference between the different types of array elements may be theoperational wavelengths range of the array elements differ. E.g. a firsttype of array elements may be configured to operate in the wavelengthrange of visible light, e.g. 400-700 nm. E.g. a second type of arrayelements may be configured to operate in e.g. the infrared wavelengthrange, e.g. 700 nm-200 nm. E.g. a third type of array elements may beconfigured to operate in e.g. the Ultra Violet wavelength range, e.g.200-400 nm. Each array element may be designed such that it operatesoptimally for its respective wavelength range. Illumination and/ordetection optics may be optimized for the respective wavelength range,the detector/sensor/pixel array may be optimized for the respectivewavelength range, etc.

In line with previously discussed embodiments, each individual arrayelement 904 may have certain actuators for a fine positioning of thearray element 904 in the array such that an advantageous positioningwith respect to metrology structures on the substrate is provided.

The array of array elements 904 may be provided in the metrology tool ata fixed position, while the metrology tool is configured to move thesubstrate 902 with, for example, a moveable substrate table. The arrayof array elements 904 may also be moveable in the metrology tool. Forexample, the array of array elements 904 may be moveable such that onegroup of array elements 904 is positioned at a location that is e.g. acentral position of the substrate table with the substrate 902. Therebythe centrally located group of array elements 904 can be used formeasurements over the whole substrate 902 based on movements of thesubstrate 902 with respect to the array of array elements 904.

In the example described above in relation to FIG. 6, the AOTFs may beused as fast shutters to provide an improved acquisition time. Inaddition to this, or alternatively, a plurality of AOTFs may be used toprovide a wavelength selection at multiple wavelengths such thatmultiple wavelengths can be detected by different detection opticalsensors. The detection by the different detection optical sensors may becarried out at the same time. Providing multiple detection opticalsensors detecting different colours can reduce acquisition time andincrease throughput. Further, using multiple wavelengths can providemore information about the targets which may have different levels ofsensitivity to the different wavelengths in different circumstances, asknown in the art.

As described above, a radiation source may be a broadband light sourceand may be capable of generating radiation in a relatively widewavelength range, for example, from 200 nm to 2000 nm. In some of theabove described examples, the sensors may be configured to only detect asingle wavelength at a time with the wavelength being selected by theacousto-optic-tunable filters described above in relation to FIG. 6, orotherwise. However, this may mean that only a very small fraction of thegenerated light radiated by a source is used at any one time. In someexamples it may be beneficial to use the different wavelengths of theradiation source in parallel rather than selecting a single wavelengthat a time. By using the wavelengths in parallel, the usable source powermay be effectively increased. This can result in a reduction in therequirements of the radiation source, which are typically costly, or canallow the throughput to be increased.

FIG. 10 shows a schematic representation of a metrology tool 1000 fordetermining a parameter of interest of a structure fabricated on asubstrate. FIG. 10 merely provides a schematic representation of afurther inventive concept and aspects of the metrology tool will besimilar to those known in the art or described above and may not bedescribed in detail further here.

The metrology tool 1000 may comprise: an illumination optical system forilluminating the structure with illumination radiation. The illuminationoptical system may comprise: a broadband radiation source 1052 and aplurality of filters 1002 a-c for filtering the broadband radiationemitted from the broadband radiation source 1052. Each filter 10002 a-cmay be configured to provide a filtered output comprising one or morewavelengths, typically a single wavelength, of illumination radiationfor illuminating the structure. The illumination system may also includea plurality of optical paths 1062 a-c for transmitting a respective oneof the filtered outputs to the structure.

The metrology tool 1000 may also include a detection optical systemcomprising a plurality of detection optical sensors 10102 a-c. Theplurality of detection optical sensors 10102 a-c may be arranged todetect illumination radiation scattered by the structure (not shown)provided by an associated one of the plurality of filters 1002 a-c.Hence, a first filter 1002 a may be configured to select one or morewavelengths which are delivered to the structure and the scattered lightdetected by a first one of the detection optical sensors 10102 a. Asecond filter 1002 b may have a similar relationship with a secondsensor 10102 b for a second wavelength and so on.

The radiation source 1052 may be a broadband radiation source asdescribed previously in relation to FIGS. 4 and 6, for example, and maycomprise a coherent radiation source emitting illumination radiationthat is received by the first or each of the plurality of filters 1002a-c. The coherent radiation source comprises a laser emitting whitelight.

The filters may be ATOFs as are well known in the art, however, otherfilters may be used. Other filters may include one or more of: a filterwheel, a gradient filter or a prism to select the different colours. Thefilters 1002 a-c, particularly AOTF filters, may be arranged in seriesor sequential relation such that unfiltered radiation passing through afirst filter 1002 a enters the next filter in line, e.g. 1002 b. Thus,each of the AOTFs may be configured to filter out a single wavelength(or narrow range of wavelengths) and pass the remaining light to thenext AOTF in the series.

The filtered wavelength is used to illuminate the target via an opticaldelivery path 1062 a which may include one or more optical fibres, asshown. The optical delivery path may be coupled directly to the filtersor may include one or more one or more re-directing element, such as amicro-mirror, to selectively redirect the filtered light to the targetvia the transmission medium such as a fibre optic. In the example shownin FIG. 10, there are three filters arranged in series, however, it willbe appreciated that other arrangements may be possible. Otherarrangements may include more or less filters, however, there willtypically be at least two filters to allow the selection of differentwavelengths. In some embodiments, the AOTFs may be arranged in parallel.In such a case, the broadband radiation may be split by one or more beamsplitters and sent down parallel paths, with each parallel pathcomprising a separate filter. Each filter may filter a differentfrequency. However, it will be appreciated that this may lead to a lowerefficiency.

An optical sensor similar to that shown and described either of FIG. 4or 6 may be used in the arrangement. Thus, the sensor may be a singlesensor and may be capable of detecting the radiation of differentwavelengths. In other embodiments, the sensors may include multipledetectors, e.g. camera chips, which are capable of each measuringspecific ranges of wavelengths. In this case, the optical system maysplit the illumination radiated from the target to provide a beam foreach detector.

In some examples, it may be advantageous to use multiple opticaldetection systems similar to those disclosed in FIGS. 9a and 9b to allowfor parallel metrology and an associated increased throughput. Thus,each sensor shown in FIG. 10 may form one of a plurality of sensorswhich are each arranged to detect the illumination radiation scatteredfrom a plurality of separate areas on the substrate, as previouslydescribed. Thus, each sensor 10102 a-c may be aligned with an individualdie, for example.

Each of the filters 10102 a-c may provide radiation of a predeterminedwavelength to a single structure. Thus, the first filter may provideillumination having a first wavelength to a first structure, and asecond filter may provide illumination having a second wavelength to asecond structure. The first and second structures may be provided onrespective dies.

In some examples, the filters 10102 a-c may be adjusted to cycle throughthe different wavelengths such that each of the respective targets isilluminated by the different wavelengths in turn. Thus, after apredetermined acquisition time, the first filter 1002 a may be adjustedto provide a second wavelength λ2 to the first structure, and the secondfilter 1002 b may be adjusted to provide the first λ1 (or a third λ3 orfurther) wavelength to the second structure, and so on. In this way, thedifferent structures are exposed to each of the wavelengths in turn andthe scattered light may be detected by the sensors to provide a range ofwavelength specific data. Thus, in some examples, the AOTFs may beconfigured to cycle through a plurality of predetermined wavelengthssuch that the wavelengths detected by the sensor or groups of sensorscan receive all of the desired wavelengths in a time separated manner.In the example of FIG. 10, there are three sensors 10102 a-c, each ofwhich may be used to provide wavelengths λ1, λ2 and λ3 in turn. It willbe appreciated that there may be any number of different wavelengthsfrom 2 to N, as required. The adjustment of the filters may be achievedwith the signal generators 1056 a-c as described above and known in theart.

In relation to FIGS. 9a and 9b , the filters 1002 a-c and sensors 10102a-c may be provided in any arrangement within the array. Hence, thearray may comprise groups of sensors each measuring the same ordifferent wavelengths at any given time. Thus, the sensors shown in FIG.10 may comprise a plurality of sensors in which each of the sensors inthe plurality of sensors are provided at a different location in respectof the wafer. There may be a first plurality of sensors measuring λ1, asecond plurality measuring λ2, and a third plurality measuring λ3, forexample.

The radiation source may be polarised or unpolarised, as describedabove. In the case of an unpolarised light source, both linearpolarisations may be used simultaneously if they are distributed overtime in a similar fashion to the selection of the AOTF wavelengthsdescribed above.

It is envisaged that providing the arrangement of FIG. 10 or itsvariants described above, and/or an array of array elements 904 may bedone in conjunction with any of the features described in connectionwith the above described examples and embodiments, where possible. It isalso envisaged that the array of array elements may be implemented insome systems which do not include all of the features of some of thedescribed examples. Thus, for example, a metrology tool having anillumination optical system for illuminating the structure withillumination radiation may or may not share an optical element with anyof the detection optical systems where there is an array of detectionoptical elements and/or illumination optical systems.

Further embodiments are provided in the subsequent numbered clauses:

1. A metrology tool for determining a parameter of interest of astructure fabricated on a substrate, the metrology tool comprising:

an illumination optical system for illuminating the structure withillumination radiation under a non-zero angle of incidence;

a detection optical system comprising a detection optical sensor and atleast one lens for capturing a portion of illumination radiationscattered by the structure and transmitting the captured radiationtowards the detection optical sensor,

wherein the illumination optical system and the detection optical systemdo not share an optical element.

2. The metrology tool according to clause 1, wherein at least part of anoptical axis of the illumination optical system is substantiallyparallel to an optical axis of the detection optical system.

3. The metrology tool according to clause 1 or 2, wherein at least partof the illumination optical system is positioned radially outwards froma radial extent of the detection optical system.

4. The metrology tool according to any preceding clause, wherein theillumination optical system comprises a plurality of discrete opticalpaths.

5. The optical system according to clause 4, wherein at least two of theplurality of optical paths are diametrically opposed.

6. The metrology tool according to any preceding clause, wherein theillumination optical system comprises at least one mirror for reflectingillumination radiation towards the structure.

7. The metrology tool according to clause 6 when dependent on directlyor indirectly on clause 4 or 5, comprising at least one mirror in one ormore of the plurality of optical paths.

8. The metrology tool according to clause 6 or 7, wherein the at leastone mirror is configured to direct the illumination radiation onto thestructure through a volume between at least one lens of the detectionoptical system and the substrate.

9. The metrology tool according to any of clauses 6 to 8, wherein the atleast one mirror is configured to direct radiation having a plurality ofwavelengths in a range from 200 nm to 2 μm to substantially the samepoint on the substrate.

10. The metrology tool according to any of clauses 6 to 9, wherein theat least one mirror is one of an elliptical or a parabolic mirror.

11. The metrology tool according to any of clauses 6 to 10 wherein theat least one mirror has a reflectivity greater than 90% across a rangeof wavelengths of the illumination radiation.

12. The metrology tool according to any preceding clause, wherein thedetection optical system has a total transmissivity of greater than 90%.

13. The metrology tool according to any preceding clause, wherein thedetection optical system comprises 5 optical elements or fewer.

14. The metrology tool according to any preceding clause, wherein thedetection optical system comprises one or more of: a plano-convex lens;an aspheric lens; a bispherical lens; a bi-aspheric lens and a longworking distance objective.

15. The metrology tool according to any preceding clause, wherein thedetection optical system comprises a plurality of lenses, and whereinone of the plurality of lenses positioned closest to the substrate has aworking distance between the substrate and a surface of the lens of oneof: greater than 300 μm; greater than 500 μm; in a range from 300 μm to1 mm; and in a range from 300 μm to 10 mm.

16. The metrology tool according to clause 15, wherein the lens has anumerical aperture of: greater than 0.4; greater than 0.7; greater than0.9; or 0.95 or greater.

17. The metrology tool according to any preceding clause, furthercomprising a focus system comprising at least one focussing opticalsensor configured to receive zeroth order radiation reflected from thestructure and a computational imaging processor configured to determinea focus of the detection optical system based on.

18. The metrology tool according to clause 17, wherein the focussingoptical sensor comprises a quad optical sensor, arranged such that aproportion of the reflected zeroth order radiation that is sensed byeach optical sensor in the quad optical sensor is indicative of aposition of the structure.

19. The metrology tool according to clause 17 or 18 when dependentdirectly or indirectly on clause 4 wherein at least one of the pluralityof discrete optical paths is configured to receive, at least in part,reflected zeroth order radiation originating from at least one other ofthe optical paths,

-   -   and wherein the at least one of the plurality of discrete        optical paths comprises a reflective optical element configured        to direct the received zeroth order radiation towards the at        least one focussing optical sensor.

20. The metrology tool according to clause 19, wherein the at least onefocussing optical sensor is positioned radially outwards from a radialextent of the detection optical system.

21. The metrology tool according to any preceding clause, furthercomprising a polarization element arranged around an outer of thedetection optical system and configured to interact with radiationpropagating through the illumination optical system for polarizationthereof.

22. The metrology tool according to clause 21, wherein the polarizationelement is configurable to apply one of s or p polarization at aplurality of magnitudes.

23. The metrology tool according to clause 22, wherein the polarizationelement is rotatable to apply one of s or p polarization at one of aplurality of magnitudes.

24. The metrology tool according to clause 22 or 23 when dependentdirectly or indirectly on clause 4, and configured to apply one of s orp polarization at one of a plurality of magnitudes to one or more of theplurality of optical paths.

25. The metrology tool according to any preceding clause, wherein thedetection optical sensor is configured to acquire a first image based onreflected and/or diffracted radiation having one order, and furtherconfigured to acquire a second image based on reflected and/ordiffracted radiation having a further order.

26. The metrology tool according to any preceding clause when dependentdirectly or indirectly on clause 4, comprising a shutter systempositioned in at least one of the plurality of optical pathsconfigurable between an open position in which illumination radiation isallowed to pass and a closed position in which illumination radiation isblocked.

27. The metrology tool according to clause 26, wherein the shuttersystem comprises one or more acousto-optic-tunable filters.

28. The metrology tool according to clause 26 or 27, further comprisinga reference optical sensor, wherein the at least one of the plurality ofoptical paths comprises a beam splitter configured to direct aproportion of the illumination radiation to the reference optical sensorwhen the shutter system is in the open position.

29. The metrology tool according to clause 28, wherein the proportion ofthe illumination radiation is less than 5%.

30. The metrology tool according to clause 28 or 29, further comprisingan image normalisation unit configured to normalise the first and secondimages based on a reference image acquired by the reference opticalsensor.

31. The metrology tool according to any of clauses 28 to 30, furthercomprising an acquisition controller configured to control the referenceoptical sensor, the detection optical sensor and the shutter system forcapturing the first and second images.

32. The metrology tool according to clause 31, wherein a first shuttersystem is positioned in a first optical path of the illumination opticalsystem and a second shutter system is positioned in a second opticalpath of the illumination optical system, and wherein the first shuttersystem is operable for acquiring the first image and the second shuttersystem is operable for acquiring the second image.

33. The metrology tool according to clause 32, wherein the acquisitioncontroller is configured to place the reference optical sensor in anacquisition phase and to open and close the first and second shuttersystems sequentially while the reference optical sensor is in theacquisition phase.

34. The metrology tool according to clause 33, wherein the acquisitioncontroller is further configured to place the detection optical sensorin a first acquisition phase such that the first shutter system isopened and closed while the detection optical sensor is in the firstacquisition phase for acquiring the first image,

-   -   and wherein the acquisition controller is further configured to        place the detection optical sensor in a second acquisition phase        such that the second shutter system is opened and closed while        the detection optical sensor is in the second acquisition phase        for acquiring the second image

35. The metrology tool of any preceding clause, wherein the illuminationoptical system and the detection optical system has a footprint lessthan the area of a field of the substrate.

36. The metrology tool according to any preceding clause, wherein thecombination of the illumination optical system and the detection opticalsystem has at least one of an x-dimension and a y-dimension less than 26mm.

37. The metrology tool according to any preceding clause comprising aplurality of illumination optical system and detection optical systemcombinations in an array, wherein each of the plurality of illuminationoptical system and detection optical system combinations comprises anarray element, each array element having a sensing axis.

38. The metrology tool according to clause 37, wherein each arrayelement is aligned with a different field of the substrate.

39. The metrology tool according to either of clauses 37 or 38, whereinthe array elements are arranged in a two dimensional array.

40. The metrology tool according to clause 39, wherein each arrayelement has an array element footprint area on the substrate and thearray comprises a tessellation of footprint areas.

41. The metrology tool according to clause 40, wherein the footprintareas within the array are the same and one of triangular, square,rectangular or hexagonal in shape.

42. The metrology tool according to clause 41, wherein the footprintareas are arranged in a honeycomb.

43. The metrology tool according to any of clauses 39 to 42, wherein thetwo dimensional comprises m rows and n columns, wherein m and n are bothgreater than 2.

44. The metrology tool according to any of clauses 37 to 43, wherein thearray is adjustable such that the separation between adjacent arrayelements can be altered in at least one of the x direction or ydirection.

45. The metrology tool according to any of clauses 37 to 44, wherein theelements of the array sensors are tiltable with respect to the plane ofthe substrate, such that the sensing axis of each array element isadjustable so as to be perpendicular with a substrate.

46. The metrology tool according to clause 45, wherein each arrayelement comprises a tilt sensor.

47. The metrology tool according to clause 46, wherein the tilt sensoris located within the detection optical system.

48. The metrology tool according to clause 47, wherein the tilt sensoris an optical sensor.

49. The metrology tool according to clause 47 or 48, wherein the sensingelement is located within the detection optical sensor.

50. The metrology tool according to any of clauses 37 to 49, whereineach array element is rotatable about the sensing axis.

51. The metrology tool according to clause 50, wherein the element isconfigured to be rotatable about the sensing axis from a first positionto a second position, wherein the first and second positions are inanti-parallel.

52. The metrology tool according to any of clauses 37 to 51, whereineach array element comprises one or more actuators configured to movethe array elements.

53. The metrology tool according to clause 52, wherein the actuatorscomprise piezo motors.

54. The metrology tool according to any of clauses 37 to 53, furthercomprising a controller, wherein the controller is configured toposition the array elements at a predetermined pitch, wherein thepredetermined pitch corresponds to the pitch of metrology targets on asubstrate.

55. The metrology tool according to any of clauses 37 to 54, wherein thespacing of the array elements corresponds to a die pitch of a substrate.

56. The metrology tool according to any of clauses 37 to 55, wherein thepitch of the array elements in the x or y direction is one or more of16.5 mm, 26 mm, 33 mm.

57. The metrology tool according to any of clauses 4 to 54, comprising acoherent radiation source emitting illumination radiation that isreceived by a plurality of optical paths of the illumination opticalsystem.

58. The metrology tool according to clause 55, wherein the coherentradiation source comprises a laser emitting white light.

59. A metrology tool for determining a parameter of interest of astructure fabricated on a substrate, the metrology tool comprising:

an illumination optical system for illuminating the structure withillumination radiation;

an array of detection optical systems comprising a detection opticalsensor and at least one lens for capturing a portion of illuminationradiation scattered by the structure and transmitting the capturedradiation towards the detection optical sensor,

wherein each element of the array of detection optical systems isadjustable such that the separation between adjacent elements of thearray can be altered in at least one of the x direction or y direction.

60. A metrology tool for determining a parameter of interest of astructure fabricated on a substrate, the metrology tool comprising:

an illumination optical system for illuminating the structure withillumination radiation;

an array of detection optical systems comprising a detection opticalsensor and at least one lens for capturing a portion of illuminationradiation scattered by the structure and transmitting the capturedradiation towards the detection optical sensor,

wherein the array is a two dimensional array with array elementsarranged in a two dimensional array having m rows and n columns.

60a. A metrology tool according to any one of the clauses 37 to 55wherein the illumination optical system and detection optical systemcombinations forming the array elements comprise at least oneillumination optical system and detection optical system combinationthat is different from another illumination optical system and detectionoptical system combination.

60b. A metrology tool according to clause 60a, wherein the at least oneillumination optical system and detection optical system combinationthat differs from the another illumination optical system and detectionoptical system combination comprises an illumination optical systemand/or detection optical system that are configured to operate in afirst operational wavelength range that is different from an secondoperational wavelength range of the another illumination optical systemand detection optical system combination.

60c. A metrology tool according to any one of the clauses 60a and 60b,wherein groups of illumination optical system and detection opticalsystem combinations are provided and within a single one of the groupsthe illumination optical system and detection optical systemcombinations are equal to each other, while the different groups havedifferent illumination optical system and detection optical systemcombinations.

61. A metrology tool for determining a parameter of interest of one ormore structures fabricated on a substrate, the metrology toolcomprising:

-   -   an illumination optical system for illuminating the one or more        structures with illumination radiation, wherein the illumination        optical system comprises a broadband radiation source, a        plurality of filters for filtering the broadband radiation        emitted from the broadband radiation source, wherein each of the        plurality of filters is configured to provide a filtered output        comprising one or more wavelengths of illumination radiation for        illuminating one or more of the structures, and, a plurality of        optical paths for transmitting a respective one of the filtered        outputs to the one or more structures;    -   a detection optical system comprising a plurality of detection        optical sensors, wherein each of the plurality of detection        optical sensors is arranged to detect illumination radiation        scattered by the structure provided by an associated one of the        plurality of filters.

62. A metrology tool according to clause 61, wherein the plurality offilters are arranged in series, wherein each filter removes the one ormore wavelengths of illumination radiation and transmits the remainingwavelengths to the next filter in the series.

63. A metrology tool according to clause 62, wherein the filters aretuneable such that one or more wavelengths (or ranges of wavelengths)may be selected from the broadband illumination radiation.

64. A metrology tool according to clauses 63, further comprising acontroller configured to control the selection of the one or morewavelengths for each one of the plurality of filters.

65. A metrology tool according to clause 64, wherein the controller isconfigured to periodically change the selection of the one or morewavelengths so as to cycle through a plurality of different one or morewavelengths such that each of the plurality of optical detection sensorsreceives scattered light from each of the one or more wavelengths.

66. A metrology tool according to any of clauses 61 to 65, wherein theplurality of optical paths each comprise an optical fibre fortransmitting the one or more wavelengths of radiation from therespective each of the plurality of filters to the target.

67. A metrology tool according to any of clauses 61 to 66, wherein eachof the plurality of filters is an ATOF.

68. A metrology tool according to any of clauses 61 to 67 furthercomprising an array of detection optical systems each comprising aplurality of detection optical sensors and at least one lens forcapturing a portion of illumination radiation scattered by the structureand transmitting the captured radiation towards the plurality ofdetection optical sensors.

69. A metrology tool comprising:

a broadband illumination source;

a plurality of ATOFs arranged in series and configured to select one ormore wavelengths for transmission to a target structure;

a plurality of detection optical sensors, each detection optical sensorconfigured to detect radiation from one of the ATOFs.

70. A metrology tool according to clause 69, wherein the plurality ofATOFs are cycled to select different one or more wavelengths such thateach of the detection optical sensors receives illumination of differentwavelengths over time.

71. A metrology tool according to clause 69, wherein further comprisinggroups of detection optical sensors in which each of the detectionoptical sensors within a group is configured to detect the samewavelength as the other detectors in the group of detection opticalsensors.

72. A metrology tool according to any of clauses 61 to 71, wherein eachof the plurality of sensors is aligned with an individual target and/ordie of the substrate.

73. A metrology tool according to clause 68, further comprising any ofthe features provided in any of clauses 37 to 58 with or without theclauses from which they depend.

It will be appreciated that clauses 59 and 60 above, may be combinedwith any of clauses 1 to 58, in particular, the features recited inclauses 37 to 58 may be made to depend on clauses 59 and/or 60 asappropriate. It will also be appreciated that any of clauses 61 to 67may be combined with any of clauses 1 to 60.

Although specific reference may be made in this text to the use oflithographic apparatus in the manufacture of ICs, it should beunderstood that the lithographic apparatus described herein may haveother applications. Possible other applications include the manufactureof integrated optical systems, guidance and detection patterns formagnetic domain memories, flat-panel displays, liquid-crystal displays(LCDs), thin-film magnetic heads, etc.

Although specific reference may be made in this text to embodiments ofthe invention in the context of a lithographic apparatus, embodiments ofthe invention may be used in other apparatus. Embodiments of theinvention may form part of a mask inspection apparatus, a metrologyapparatus, or any apparatus that measures or processes an object such asa wafer (or other substrate) or mask (or other patterning device). Theseapparatus may be generally referred to as lithographic tools. Such alithographic tool may use vacuum conditions or ambient (non-vacuum)conditions.

Although specific reference may have been made above to the use ofembodiments of the invention in the context of optical lithography, itwill be appreciated that the invention, where the context allows, is notlimited to optical lithography and may be used in other applications,for example imprint lithography.

The skilled person will be able to envisage other embodiments withoutdeparting from the scope of the appended claims.

The invention claimed is:
 1. A metrology tool for determining aparameter of interest of a structure fabricated on a substrate, themetrology tool comprising: an illumination optical system configured toilluminate the structure with illumination radiation under non-zeroangles of incidence; a detection optical system comprising: at least onelens configured to direct a portion of illumination radiation scatteredby the structure; and a detection optical sensor configured to receivethe directed portion of illumination radiation scattered by thestructure and to generate image information of the structure, whereinthe illumination optical system and the detection optical system do notshare an optical element, and wherein at least part of an optical axisof the illumination optical system is substantially parallel to anoptical axis of the detection optical system; and a computationalimaging processor configured to: receive aberration information of theat least one lens and the generated image information; and adjust theimage information based on the aberration information.
 2. The metrologytool of claim 1, wherein at least part of the illumination opticalsystem is positioned radially outwards from a radial extent of thedetection optical system.
 3. The metrology tool of claim 1, wherein theillumination optical system comprises at least one mirror for reflectingillumination radiation from a corresponding one of the at least twofiber optics towards the structure.
 4. The metrology tool of claim 3,comprising at least one mirror in one or more of a plurality of opticalpaths.
 5. The metrology tool of claim 3, wherein the at least one mirroris configured to direct the illumination radiation onto the structurethrough a volume between at least one lens of the detection opticalsystem and the substrate.
 6. The metrology tool of claim 3, wherein theat least one mirror is configured to direct radiation having a pluralityof wavelengths in a range from 200 nm to 2 μm to substantially the samepoint on the substrate.
 7. The metrology tool of claim 3, wherein the atleast one mirror is one of an elliptical or a parabolic mirror.
 8. Themetrology tool of claim 1, wherein the detection optical system has atotal transmissivity of greater than 90%.
 9. The metrology tool of claim1, wherein the detection optical system comprises at least one of: aplano-convex lens; an aspheric lens; a bispherical lens; a bi-asphericlens and a long working distance objective.
 10. The metrology tool ofclaim 1, further comprising a focus system comprising at least onefocusing optical sensor configured to receive zeroth order radiationreflected from the structure, wherein the computational imagingprocessor is further configured to determine a focus of the detectionoptical system based on focus information received from the at least onefocusing optical sensor and to adjust the image information based on thefocus information.
 11. The metrology tool of claim 1, further comprisinga polarization element arranged around an outer portion of the detectionoptical system and configured to interact with radiation propagatingthrough the illumination optical system for polarization thereof. 12.The metrology tool of claim 1, further comprising a shutter systempositioned in at least one of the plurality of optical pathsconfigurable between an open position in which illumination radiation isallowed to pass and a closed position in which illumination radiation isblocked.
 13. The metrology tool of claim 1, wherein the illuminationoptical system and the detection optical system together have afootprint less than the area of a field of the substrate.
 14. Themetrology tool of claim 1, wherein the illumination optical systemcomprises at least two optical fibers configured to guide theillumination radiation to the structure.
 15. The metrology tool of claim1, wherein a cross-sectional diameter of a combination of theillumination optical system and the detection optical system is smallerthan 50 mm.
 16. The metrology system of claim 1, wherein the adjustingthe image information based on the aberration information comprisesenhancing a dark field image determined from the image information. 17.The metrology system of claim 1, wherein: the at least one lens isfurther configured to transmit a plurality of wavelengths of the portionof illumination radiation scattered by the structure; the at least onelens has associated aberrations in the plurality of wavelengths; and theadjusting of the image information based on the aberration informationcomprises correcting effects of the associated aberrations on the imageinformation.
 18. A metrology tool for determining a parameter ofinterest of a structure fabricated on a substrate, the metrology toolcomprising: an illumination optical system configured to illuminate thestructure with illumination radiation under non-zero angles ofincidence; a detection optical system comprising: at least one lensconfigured to direct a portion of illumination radiation scattered bythe structure; and a detection optical sensor configured to receive thedirected portion of illumination radiation scattered by the structureand to generate image information of the structure, wherein theillumination optical system and the detection optical system do notshare an optical element, wherein the illumination optical systemcomprises a plurality of discrete optical paths, and wherein at leasttwo of the plurality of optical paths are diametrically opposed whenincident on the structure; and a computational imaging processorconfigured to: receive aberration information of the at least one lensand the generated image information; and adjust the image informationbased on the aberration information.